Co-owned U.S. patent application Ser. No. 15/248,151, filed Aug. 26, 2016, entitled “FLOATING DIE PACKAGE”, (the '151 Application) naming Benjamin Stassen Cook as inventor, which application is hereby incorporated by reference in its entirety herein, describes an integrated circuit package with a cavity formed within a molded package by sublimation and use of a sacrificial material to form the cavity.
Fuses are protective elements in electrical circuits that operate by creating an open circuit under certain conditions. The purpose of a fuse varies according to the application. In some cases, fuses are used as programmable elements to reconfigure logic or memory circuits. More traditionally, a fuse creates a failsafe circuit element to protect an electrical system from overcurrent conditions that can damage wiring or other components within the system. The fuse is either reversibly or irreversibly physically changed to open a circuit during the overcurrent conditions. The open circuit stops current flow before permanent damage occurs elsewhere in the system.
Conventional fuses are composed of a metal filament that melts and creates an open circuit after a pre-determined amount of current flows through it. A fusible link has metal or conductive components connected by a sacrificial portion that melts, such as solder or a small diameter wire. Conductive traces with a small cross-section can also be used. The open circuit is created when current passes through the fuse element, causing the fuse element to heat enough that the fuse element melts or “blows”. During the melting phase, as the current begins to be interrupted and the open forms, an arc can form which vaporizes a portion of the fuse material. The arc will grow, consuming and vaporizing the fuse element, until the distance becomes too long to sustain the arc and current stops flowing. In some molded integrated circuit packages, if a fuse is included in the package, carbonization of the mold compound can occur, and filaments or filler in the mold compound may become conductive paths that continue to carry current.
Fuse material that is vaporized when a fuse blows is deposited in the local area and the probability of creating a secondary conductive path arises. To contain the vaporized material and prevent electrical conduction in the aftermath of the arc, traditional fuses are encapsulated in a non-conductive glass vial surrounding the fuse element. In higher current fuses, the fuse element is contained within a non-conductive ceramic container that is packed with quartz sand. In the case of the glass vial, the vaporized fuse material is contained by the glass with some of it coating the glass, however the vaporized material is so widely dispersed that a secondary conductive path is not formed. In the case of a sand filled fuse container, the vaporized fuse material is dispersed among the sand and the heat from the fusing arc melts the sand forming a glass that prevents current flow.
With the increasing use of integrated circuits, miniaturization of fuses plays a part in the effort to reduce product sizes and costs while still protecting circuits. Higher current fuses result in higher temperatures and additional opportunity for the regrowth of a secondary conductive path. A fuse on an integrated circuit has upper limits on the current capacity as a result of the small geometries and the cost resulting from the multiple levels of masks needed to form the fuses. To attain higher current fuses, the fuse elements are sometimes moved to the PCB where the IC is mounted, or placed off the integrated circuit die but still located inside the package. The disadvantage of creating a fuse from an element of the PCB is that when blown, the entire PCB can require replacing, a cost prohibitive repair. A more practical solution is to form a fuse within a package so that in case of a blown fuse, only the packaged part needs to be replaced. In an application using a semiconductor power device, a common overstress failure mode is a shorted device (e.g., a current path to ground). MOSFET devices forming power switches sometimes fail in a shorted state with a path to ground due to an overvoltage, an ESD strike, exceeding safe operating conditions for the device or from a defect in the manufacture of the device. These failure modes require a fuse element to protect the system.
An on-chip fuse element or fusible link, such as a bond wire encapsulated in an IC package, will often form a secondary conductive path during an overcurrent event. The secondary conductive path can form after the high current superheated arc expands and burns the surrounding mold compound to form a carbonized conductive path. The secondary conductive path sustains the arc until the device burns up sufficiently to extinguish the arc. Catastrophic damage to the device and often to the printed circuit board the device is mounted on can occur before the arc is extinguished. A need continues to exist for an improved fuse apparatus for integrated circuit packages that is able to interrupt high currents, have a low probability of a secondary conductive path forming, and fit within the semiconductor package.